A PCB for a lightweight QRP common-mode choke — rated for up to 10 W (SSB & CW) or 5 W on FT8.
This common-mode choke (CMC) is built around an Amidon FT82-43 core with a smaller FT50-43 core nested inside it. The nested arrangement yields a compact, low-loss choke with improved high-frequency behaviour compared with a single core of the same outer size — an excellent choice for QRP use where minimal dissipation and a small form factor matter.
- Outer core: Amidon FT82-43 (or Fair-Rite equivalent 5943000601)
- Inner core: Amidon / Fair-Rite FT50-43 (5943001101) placed inside the FT82-43
- Alternative (high bands only): Würth 74270118 — performs well on higher frequencies but not recommended below ~6 MHz.
- For 40–10 m common-mode suppression, many builders start with ~8–10 symmetrical turns, which are usually sufficient for typical QRP antenna setups.
- More turns increase common-mode impedance at lower frequencies, but also raise inter-winding capacitance — measure and adjust for your target bands.
- Aim for at least 20 dB of common-mode attenuation across the target band where practical.
- Target an SWR as close to 1:1 as possible after installing the choke; SWR below 1.5:1 is generally acceptable for most QRP uses.
Wire & mechanical tips
- Use enamelled copper wire sized appropriately for your power and mechanical needs — commonly 0.4–0.8 mm enamelled wire works well for QRP.
- Keep pigtails and external leads short to avoid added series inductance.
- Secure the nested cores mechanically with non-conductive tape or heat-shrink to prevent rotation or movement.
Recommended guide (detailed):
- Measuring common-mode choke performance — Lance Conry.
https://lance.conryclan.com/home/measuring-common-mode-choke-performance/
Quick checklist:
- Calibrate your VNA (or impedance meter) for the measurement band.
- Measure insertion loss / S21 in common-mode configuration to estimate attenuation.
- Measure impedance / Z vs frequency to verify target common-mode impedance.
- Verify SWR of your antenna/system with the choke installed to confirm no adverse effects.
- Iterate turns and retest until you hit the desired tradeoff between attenuation and bandwidth.
| Band | CMC attenuation (dB) | Insertion loss (dB) | VSWR |
|---|---|---|---|
| 80 m | -29.63 dB | -0.09 dB | 1.008 |
| 40 m | -32.78 dB | -0.21 dB | 1.014 |
| 30 m | -33.96 dB | -0.29 dB | 1.019 |
| 20 m | -34.39 dB | -0.32 dB | 1.026 |
| 17 m | -34.16 dB | -0.24 dB | 1.037 |
| 15 m | -33.45 dB | -0.11 dB | 1.047 |
| 12 m | -32.24 dB | +0.11 dB | 1.059 |
| 10 m | -30.62 dB | +0.37 dB | 1.073 |
You can order this PCB from JLCPCB or any other service using the provided Gerber archive.
Download Gerber
./gerbers/qrp-cmc-pcb-v1.1-gerber.zip
Use the following manufacturing parameters when placing the order:
- Board thickness:
1.6 mm - Layers:
2 (two-layer) - Material:
FR-4 - Copper weight:
1 oz (35 µm) - Solder mask: Chose your preferred